2 Kings 20:5

Turn again, and tell Hezekiah the captain of my people
The king of them, as the Targum:

thus saith the Lord God of David thy father, I have heard thy prayer,
I have seen thy tears; (See Gill on Isaiah 38:5)

behold, I will heal thee;
instantly, miraculously; and none but God could heal him, his disease being in its kind mortal, and he had been told from the Lord that he should die:

on the third day thou shalt go up unto the house of the Lord:
the temple, to give thanks for his recovery; and this he should do on the third day from thence; so soon should he be well, which would show the cure to be miraculous.